Noise limiter



0i 27. 1942. s. x.. GRUNDMANN NOISE LIHITER Filed ay 21.1941

200 A1C-P l l T... Sb

/17' Df7: WILL/970B nventor attorney Patented Oct. 27, 1942 UNITEDSTATES PATENT co,F-rfifcEffN NOISE LIMITEE.'

Gustave L. Grundmann, Westmont, N. J., assign,

or to Radiol Corporation of America, a corporation of DelawareApplication May 27, 1941, Serial No. 395,371

(Cl. Z50- 20) 6 Claims.

My invention relates to noise limiters for radiol tive for noisereduction in a sound channel having a wide pass band such` as that in atelevision receiver.

By the series type limiter is meant a limiter of the general type4described in Kaar Patent 2,207,- 587 wherein the gate Voltage maintainsa current flow through a diode and an output resistor in series whilethe soundl signal is applied to this series combination with 'a-polarity opposed to that of the gate voltage. If noise pulses in thesignal exceed the gate voltage,vthe ow of diode current ceases for thedurationfof the noise pulse thus limiting its amplitude.- l

An object of my invention is to provide an improved and simplified noiselimiter circuit of the above-described type in which the gate voltagevaries with changes in the average carrier.

A further object of theinvention is to provide an improved andsimplified noise limiter circuit of the above-described type in whichthe `gate voltage varies in accordance with changes-in the averagecarrier and in which the gate voltage also substantially follows theuseful modulation envelope of the sound modulatedY carrier wave.

The invention will be better understood from the following descriptiontaken in connection with the accompanying drawing in which Figure 1 is acircuit diagram showing the wide band sound channel of a televisionreceiverembodying my invention,

Figure 2 is a circuit diagram similar to Fig. l but showing anotherembodiment ofthe invention, and

Figure 3 is a circuit diagramV `showing another embodiment of theinvention applied to a narrow band radioreceiver such'as a policereceiver.

Fig. 1 shows the invention applied' to theV sound channel of atelevision receiver.` `The 'receiverv `map comprise a first detector anda tunable oscillator indicated at IIJ for converting `the incomingpicture signal and accompanyingy sound signal to intermediate frequencysignals.

The soundsignal is supplied to an intermediate frequency amplifierindicated by the rectangle II which has a pass band several times thewidth of the vfrequency band occupied bythe sound signal being passed.-vvFor example, thev pass band may be 200 k. c. and the vsignall band-8 for 10 k. c. One I. F. amplifier tube and a'couplingV .r transformer areindicated generally I by y dotted lines inthe rectangle II.

The I. F. amplifier output is supplied through-a 1 coupling transformerI2 to the second detector4v This detector, in the example villustrated,4

I3. comprises a diode I4 and a load resistor' I6 connected in serieswith each other and acrossthe secondary of the transformer I2. Theresistor IB is shunted by the usual bypass condenser Il for f bypassingthe I. F. carrier wave.

The receiver is provided with a suitable 'auto-f i matic volume control`or A. V. C. circuit operating on average carrier. In the' exampleillustrated, the average carrier voltage appearing across the detectorload resistor I6 is applied to-one-ormore of the I. F. amplifier tubesthrough a resistor-v I5. The usual A. V. C. filtery condenser isshown'at 20.

The noise limiterA comprises a diode I8 and anY output resistor I9connected in series with each other and across the loadr'esistor I6through a coupling condenser 2| and through a bypass condenser 22 ofthe' voltage divider 23.- Thus the A. C. sound signal' is applied acrossthe series I8-I9. As explained hereinafter,

denser 24 connected thereacrossA for causing the gate or limiter voltageto follow the useful modulation envelope.

` A flow of direct current is maintained through the limiter diode IB bymeans of a voltage drop produced `by the automatic volume control cur-vIn the embodiment rent or a portion thereof; of Fig. l, this voltagedrop is obtained from a relsistor 26 through which plate current flowsto oneu or more of the I. F. amplifier tubes controlled'by the A. V. C.circuit. The said voltage drop is applied to the series combination|8-I9 throughV a D.'C. connection comprising a high'impedance resistor2'I. and the left hand vportion of the Voltage divider 23,' this portionhaving the `usual bypass condenser 28 thereacross.

The resistor 26 may be the usual filter resistor for the plate supply toone or more of the amplifier tube plates. A filter' condenser isindicated From the foregoing it will vbe-apparent that there is appliedto the limiterl diode 18a gatev Voltage which Varies withV changes intheaverage carrier and that this voltage and'thesignal voltcircuitIii-24 causes the gate voltage to follow the luseful modulationenvelope-fof the-sound modulated carrier wave. The time constant of thisR-C circuit is such that it has a cut-off just beyond the highestfrequency component of the sound signal if the signal is to appear atthe output circuit without any attenuation of the higher frequencysignal components.

However, as described and claimed in the copending Holmes applicationSerial No. 395,346, led May'27,4 1941, it may be desirable to make thetime constant of the R--C network I9--24 such that some of the higherfrequency signal `components are attenuated. This may be de-V sirablefor tone control purposes and/or for complementary attenuation in asystem transmitting a pre-emphasized signal. In that case the gatevoltage follows the modulation only approximately and some portions ofthe envelope representative of higher frequency components areattenuated.

In Figure 1, the values of certain elements have been indicated, merelyby way of example, in ohms, megohms, and micromicrofarads.

The embodiment of the invention shown in Fig. 2 is similar to that shownin Fig. 1, thle difference being that the average carrier change in thegate voltage for the noise limiter in Fig. 2 isobtained from `the'screengrid circuits of the I. amplifiers vwhich have an A. V. C. controlthereon.` In Figs. 1 and 2,-like parts are indicated by the. samereference characters.

The average carrier variation of the gate voltage appearsacross a screengrid resistor SI whichis bypassed by the usual filter condenser 32. Oneor more amplifier tubes controlled by the A. V. C. circuit may havetheir screen grids supplied by the operating voltage through theresistor SI. lIf a plurality of tubes is so supplied, the resistance ofresistor 3| ordinarily should be less than the A109,000 ohms indicatedon the drawing. v l v The operation of lthe kcircuit of Fig. 2 issimilar to that of the circuit previously described. If theincomingsignal fades, the control grid of the I. F'. amplifier tubebecomes lessnegative with the-result vthat the screen grid currentincreases and the gate voltage on the plate of the limiter diode I8becomes less positive whereby the limiter diode becomes nonconducting ata lower value of signal voltage.

The circuit of Fig. 2 has an advantage over that of Fig. 1 inl thatthere is a degenerative action in the screen grid circuit whereby thechanges in screen grid voltageare nearly linear with respect to changesin average carrier and, therefore, follow the output of the lineardetector I4 rather closely.

It will be'y understood that an important feature of the. circuit inFig. 2, as well as in Fig. l, is that the gate voltage. is caused tofollow the useful modulation envelope as well as theV average carriervariations by giving the R-C circuit Iii-24 the proper time constant.

In Fig. 3, there is shown anv embodiment of the invention in which theaverage carrier variations of the gate voltage are obtained by means ofan extra tube, not a signal amplifier tube, which is controlled by theA. V. C. voltage. Also, in Fig. 3, the invention is shown applied to asound receiver, such as a police receiver, having a narrow band soundchannel and the gate Voltage in the example illustrated is not made tofollow the modulating envelope. Like parts in Figs. 1 and 3 areindicated by the same reference characters.

AThe receiver may comprise a rst detector and an oscillator indicated at40 for supplying an I. F. signal to an amplifier 4I connected through atransformer to the second detector I3. The detector I3 comprises a diodeI4 having a load resistor 42 shunted by a condenser 43.

The noise limiter connections are substantially the same as in the othergures except that the average carrier variations in the gate voltage areobtained from across a resistor 44 which supplies plate voltage to theabove-mentioned extra tube indicated at 46.

The tube 46 has a control electrode connected through a resistor 41 to apoint on the A. V. C. circuit whereby the current through the plateresistor 44 will vary with the A. V. C. voltage and thus' vary the gatevoltage applied to the limiter diode I8 in accordance with changes inaverage carrier.

In this embodiment of the invention, it may be desirable to provide acondenser 5I which may be switched across the output resistor I9 forattenuating the higher frequency components of the sound signal for tonecontroll and/or additional noise reduction., It will be noted that thecondenser 5I has substantially greater capacity than the condenser 24 inthe circuit of the other figures where the gatevoltage is made to followthe modulation envelope.

From the foregoing, it will be apparent that an important feature of thepresent invention is the provision of a simplified noise suppressorcircuit of the series diode type in which the gate voltage follows boththe average carrier and the useful modulation envelope, the circuit alsohav- ;,ing the desirable feature of utilizing the full voltage output ofthe second detector.

I claim as my invention:

l. A radio receiver having a sound channel which includes an amplifier,a detector supplie-d ,from said channel and having a load resistoracross which the demodulated signal appears, a noise limiter of theseries diode type connected across at least a portion of said loadresistor, said limiter comprising a diode and an output resistor inseries, an automatic volume control signal components passed by saidchannel, said channel including an amplifier, a detector supplied fromsaid channel and having a load resistor across which the demodulatedsignal appears, a noise limiter of the series `diode type connectedacross substantially the entire portion of said load resistor, saidlimiter comprising a diode and an output resistor in series, anautomatic volume control circuit for said amplier, means controlled bysaid circuit for applying to said limiter a gate voltage which varieswith average carrier, and means for also causing the gate voltage tovary with the useful modulation envelope of the received carrier wave.

3. A radio receiver having an intermediate frequency amplifier with apass band which is several times the width of the frequency bandoccupied by the signal components passed by said channel, a detectorsupplied from lsaid ampliiier and having a `load resistoracross-whichthe demodulated signal appears, a noise limiter of A the series diodetype connected across at least a portion of said load resistor, anautomatic volurne control circuit for controlling the gain of saidamplifier, said limiter comprising a diode and an output resistor inseries, a resistor through which voltage is supplied to an electrode ofan amplifier tube in said amplifier, and .means for applying the voltagedrop appearing across said last resistor to said limiter as at least aportion of its gate voltage whereby the gate voltage varies with averagecarrier.

4. The invention according to claim 3 wherein the output resisor of thelimiter is shunted by capacity of such value that the gate voltage alsofollows the useful modulation envelope of the received carrier wave.

5. A radio receiver having a sound channel including an amplifier tube,a. detector connected to be supplied with signal from said channel, saiddetector having a load resistor across which the demodulated signalappears, a noise limiter. connected across at least a portion of saidload resistor, an automatic volume control circuit for. controlling thegain of said amplifier tube, la' resistor through which voltage isapplied to screen grid electrode in said amplifier tube, and means forapplying the voltage drop appearing across said last resistor to saidnoise limiter as at least a portion of its gate voltage whereby the gatevoltage varies With average carrier. y

6. The invention according to claim 5 wherein said limiter is of theseries type which has an output resistor shunted by capacity of suchvalue that the gate voltage also follows the useful modulation envelopeof the received carrier wave. l

GUSTAVE L. GRUNDMANN.

